1. Field of the Invention
The present invention relates to a charge pump circuit.
2. Description of the Related Art
Recent electronic apparatuses such as cellular phones and PDAs (Personal Digital Assistants) have incorporated devices which require driving voltages larger than battery voltages, such as LEDs (Light Emitting Diodes) used as back lights for liquid crystal displays. For example, these small-sized information terminals frequently employ Li-ion batteries which output voltages of about 3.5 V in general and output voltages of about 4.2 V even at fully-charged states, while LEDs require voltages larger than such battery voltages, as driving voltages therefor. In cases where a voltage larger than a battery voltage is required as described above, a charge pump circuit and a switching regulator are used for boosting the battery voltage for generating a voltage required for driving LEDs.
A charge pump circuit generates an output voltage by multiplying an input voltage by a predetermined voltage-boosting ratio. For example, in the case where the battery voltage is 3 V and the voltage-boosting ratio is 2, the output voltage is fixed to 6V. Accordingly, when a load circuit requires a driving voltage lower than 6V, there has been a need for inserting a power transistor in the input side or the output side of the charge pump circuit to constitute a linear regulator for adjusting the ON resistance thereof for adjusting the output voltage. For example, a relating technique is described in Japanese Patent Application (Laid Open) No. 2000-262043.
In the case of utilizing the technique described in Japanese Patent Application (Laid Open) No. 2000-262043, there is a need for using a power transistor, thereby increasing the number of components of the circuit and the area of the circuit.